The following functions convert JS values to various types. They can be safely applied to jsvals of any type. They may return new objects. For example, JS_ValueToObject(cx, s) where s is a string creates a new String wrapper object. These functions may call JavaScript methods. For example, JS_ValueToString(cx, obj) may call obj.toString().

Fast, unchecked type-casting macros. These macros must not be applied to values that are not known to be the right type. Like C casts, they may cause crashes if applied to incorrect values. They never create new objects or call into JavaScript code.

{{template.Jsapi_minversion_inline("1.8 (not yet released)")}} If an object contains references to other GC things that are not stored in SpiderMonkey data structures ("slots"), it must implement the JSTraceOp hook to enable the garbage collector to traverse those references. Otherwise the garbage collector will not find all reachable objects and may collect objects that are still reachable, leading to a crash. (In SpiderMonkey 1.7 and earlier, the JSMarkOp hook is used instead. This will be deprecated when SpiderMonkey 1.8 is released.)

The tracing APIs are used by the garbage collector and JSTraceOp hooks. JSAPI applications may also use them to examine the object graph. (For example, these APIs support very smooth integration between the JS garbage collector and other garbage collectors.)

The character data for external strings is stored in memory provided by the application. Applications can use this to avoid copying data back and forth between SpiderMonkey's heap and application memory.